A closed loop or feedback fuel injection control system determines a proper air-fuel ratio and constantly monitors the exhaust to verify the accuracy of the air-fuel mixture setting. When an exhaust sensor detects no oxygen content in engine exhaust, the feedback fuel injection control system undergoes a procedure to maintain an ideally combustible air-to-fuel ratio, namely a stoichiometric air-fuel mixture, by correcting itself so as to bring the oxygen concentration back to proper levels. Since the feedback fuel injection control system tries to maintain the air-fuel mixture setting even during acceleration, which requires high engine power, the engine can not provide a desired, or sufficient power under highly loaded engine operating conditions if the feedback fuel injection control takes place over the entire zone of engine operating conditions. For this reason, the feedback fuel injection control system is usually designed to interrupt or halt the feedback fuel injection control in a specific engine operating condition zone in which the engine operates with high load and, in a specific zone (which is referred to as a high-load forced fuel increase zone), an intentionally increased amount of fuel is forcibly delivered.
Typically, since such a forced fuel increase control is conducted for only a short time period immediately after every change of an engine operating condition from the feedback fuel control zone to the high-load forced fuel increase zone, the engine is apt to be subjected to a decrease in fuel economy, or mileage.
To eliminate such a decrease in fuel mileage, some fuel control systems are designed to halt fuel increase control for a predetermined time period (which is hereinafter referred to as a fuel injection delay time) after the change of an engine operating condition from the feedback control zone to the high-load forced fuel increase zone. Such a fuel control system is known from, for instance, Japanese Patent Application No. 51-83181, entitled "Air-Fuel Ratio Feedback Control System For Internal Combustion Engine," filed on July 12, 1976, and laid open as Japanese Unexamined Patent Publication No. 53-8427, on Jan. 25, 1978.
However, the fuel control system described in the above publication encounters the problem that because the interruption or halt of forced fuel increase takes place even when an engine operating condition changes from a low speed operating condition to a high-speed, high-load operating condition and, accordingly, instantaneously requires a very rapid power increase during, for instance, a quick acceleration, the engine its running ability is temporarily deteriorated.